1. Field of the Invention
This invention relates to a telescope optical system which is constructed so that a plurality of prismatic telescope optical systems with different magnifications for observation, each having an objective system, an image erecting prism system, and an eyepiece system, are practically equal in overall length, irrespective of the different magnifications.
2. Description of Related Art
FIG. 1 shows an example of the arrangement of a conventional prismatic telescope optical system. As seen from this figure, the conventional prismatic telescope optical system is constructed with an objective system 1, an image erecting prism system (using Porro prisms) 2, and an eyepiece system 3. In the case of a binocular optical system, two such optical systems are arranged in parallel. Although prismatic telescope optical systems (or binocular optical systems) vary in performance, it is common practice that an optical system which is high in magnification for observation (which is hereinafter referred simply as to magnification) is relatively large in size, whereas an optical system having a low magnification is smaller. This is because an increase of magnification requires the objective system to have a larger focal length. If, however, optical systems vary in appearance and shape according to their magnification, differing parts must be used in accordance with the magnification of the optical system and thus mass production becomes difficult. This situation exerts an adverse influence on costs. It is therefore favorable that the overall lengths of the optical systems are equal regardless of their magnifications.
In the past, the magnification of a prismatic telescope (or of a pair of binoculars) has been changed by any of the following techniques of:
(1) replacing the objective system (but not the eyepiece system) with another to change the focal length of the objective system without changing the focal length of the eyepiece system, PA0 (2) replacing the eyepiece systems (but not to the objective system) with another to change the focal length of the eyepiece system without changing the focal length of the objective system and PA0 (3) replacing the objective system and the eyepiece system with others to change the focal lengths of the objective system and the eyepiece system.
According to the first and second techniques mentioned above, if, between a plurality of optical systems having different magnifications, prism units are identical in length and refractive index, the overall lengths of the optical systems will vary as the magnifications of the optical systems are changed. With the third technique, however, it is possible to make the overall lengths of the optical systems equal, even though the magnifications are changed.
These conventional techniques, however, involve various problems as follows:
The first technique is such that if the magnification of an optical system is made high, the focal length of its objective system will increase and thus the overall length of the optical system will also increase. This makes it impossible to equalize the respective overall lengths of a plurality of optical systems having different magnifications so as to use common machine parts. If, in particular, common optical elements cannot be used , parts for the exclusive use of each of the optical systems must be prepared, thus causing a considerable increase in cost because of an extra expense for molds, for example. Furthermore, if a plurality of optical systems are not be constructed to have identical overall lengths, the design of their respective appearance must be modified. If, for example, a balanced design is obtained in an optical system having a low magnification, the objective system of this optical system when changed to a high magnification becomes greater in length, with the result that the balance of the entire optical system is destroyed. Conventionally in order to make the overall length of the optical system unchanged even when the focal length of the objective system is increased, a negative lens system is added to the objective system or a cemented lens is separated. This arrangement, however, is attended with an increase in the number of lenses and difficulty in assembly, and forms a chief cause of higher raising costs in the manufacturing process.
The second technique is such that since the eye-piece system having a large number of lenses must in general be designed so that it is for the exclusive use of each of a plurality of optical systems, expenses for jigs and tools are increased, causing high cost. Thus, conventionally in order to increase the magnification of an optical system, the focal length of the eyepiece system is reduced. However, the result is that the difficulty of correction for aberration is enhanced and the number of lenses increases to cause a rise in cost. A reduction of the focal length of the eyepiece system makes the position of the eyepoint liable to be lower, and as such the optical system is hard to handle. Moreover, it is generally difficult to make the overall lengths of the optical systems equal by changing only the focal length of the eyepiece system. If the overall lengths are forced to coincide with one another, the performance of the entire optical system is liable to deteriorate, and it becomes necessary to enlarge the number of lenses to correct this deterioration. Consequently, the entire optical system requires the formed parts to be further modified, bringing about even higher costs.
The third technique increases the number of degrees of design freedom of an optical system and facilitates the improvement of its performance. However, lens elements are designed to correspond to each of a plurality of optical systems, and thus expenses for jigs and tools are raised causing higher costs. If the overall lengths of the optical systems are made identical so that machine parts are used for any of the optical systems, it is required that the focal length of the eyepiece system be reduced as the magnification of the lens system is increased. Hence, the number of lenses is enlarged to correct for aberrations with the result being higher, a lower position of the eyepoint, and harder to handle optical system.